BELGRADE (Reuters) - Hundreds of Serbian ultra-nationalists protested on Tuesday against U.S. Vice President Joe Biden's visit to Belgrade by chanting their support for Donald Trump.

"Vote for Trump! Vote for Trump!" the protesters, wearing T-shirts displaying an image of the U.S. Republican candidate, shouted as they gathered near the Serbian presidency building.

Biden was on a one-day visit to Belgrade before traveling to Kosovo to encourage both countries to do more to normalize their relations.

The United States is highly popular among Kosovars, who regard Washington as their savior for the 1999 air strikes that halted killings of ethnic Albanians by Serbian forces waging a counter-insurgency war.

But resentment remains high in Belgrade over NATO's air strikes, whose vestiges can be seen in battered ex-Defense Ministry buildings in the center of the Serbian capital.

Kosovo declared independence from Serbia in 2008 but Serbia does not recognize its former southern province as sovereign.

"Trump is the alternative to globalization. He will destroy old centers of power in the United States and he is a supporter of Russia," Vojislav Seselj, head of Serbia's ultra-nationalist Radical Party, said when asked why he was backing Trump.

Biden, who was meeting government officials in another part of town, offered condolences for the Serbs who lost their lives in the air strikes, the first high-ranking U.S. official to do so.

"I'd like to express my condolences to the families of those whose lives were lost in the wars of the 1990s, including those killed as a consequence of the NATO air strikes," Biden told reporters before taking a walk with Prime Minister Aleksandar Vucic in a picturesque borough of Belgrade, under high security.

Biden, a Democrat, said on Monday Trump's remark that President Barack Obama had founded Islamic State had increased threats to the safety of U.S. troops in Iraq.

Trump's volatile campaign, which has included calls for a border wall with Mexico to keep out immigrants and a temporary ban on Muslims entering the United States, has pricked the interest of some right-wing and nationalist leaders abroad.

Seselj, who was acquitted in March of war crimes by the U.N. tribunal in The Hague, urged Serbian-Americans to vote for Trump in the November U.S. presidential election.

Victims of atrocities in Bosnia's 1992-95 war reacted with dismay in March to the acquittal of Seselj, who was accused of stoking murderous ethnic hatred with fiery rhetoric in the conflicts that accompanied federal Yugoslavia's break-up into seven successor states and killed 130,000 people.

Seselj's Radicals are the third largest party in the Serbian parliament.